Optical fibers have been increasingly used in recent years downhole in oil, gas or water re-injection wells as a conduit for retrieving data/signals from various sensors/tools and/or for sending data/signals to sensors/tools located within the well.
To date, an optical fiber can be deployed in an oil or gas or water injection well by a number of methods. Some of these conventional methods provide advantages such as lower cost of installation, low risk on installation, easier access to the zone of interest to be measured or better protection of the fiber against chemical attack or mechanical damage, but each of them have disadvantages which can mitigate the advantages they provide.
For example, Tubing Encased Fiber (TEF) Fixed Fiber Deployment has been used for a number of years by various organisations, and involves fixing an optical fiber inside a protective device which may be a tube, housing or solid encapsulation. The tube is then itself fixed to the outside of the production tubing, casing or other in-well tubulars, at the surface of the borehole. The TEF is then deployed or run into the borehole with the in-well tubulars.
Alternatively, the optical fiber is mounted in a micro-tube of a few mm or less diameter, which in turn is mounted inside an external package, which typically could be a ¼″ or ⅜″ diameter steel or alloy tube commonly used for various downhole applications. An encapsulation material is also commonly used to further protect this package during installation/running into the well. The optical fiber may be suitably protected by, for example, scavenger gel, or a fiber coating of metal or carbon, or other means in order to exclude hydrogen, water or other undesirable substances from the vicinity of the fiber.
Another alternative method of deploying an optical fiber downhole is to run in a suitable tube attached to the in-well tubular (such as production tubing), and to pump in the optical fiber into the suitable pre-deployed tube at a later date using a gas or a liquid to carry/transport the fiber through the tube; this method is referred to as Pumped Fiber Deployment. A number of existing patents cover various aspects of such pumped fiber deployment, such as U.S. Pat. No. 6,532,839, U.S. Pat. No. 6,557,630, U.S. Pat. No. Re 37,283, U.S. Pat. No. 6,268,911 and U.S. Pat. No. 5,892,860.
Advantages, where feasible, to install the external mounting for the fiber in the well, and to insert the optical fiber later, include the ability to replace failed or poor fibers with new items. Such new items may be devices not available at the time of initial installation.
Disadvantages of such pumped fiber deployment (i.e. insertion of the fiber subsequent to initial installation) include the risk of damage to the fiber, and logistical difficulties, e.g. in the case of subsea wells.
Previously-used pumped fiber deployment methods include pumping the fiber in with water or some other liquid. A particular disadvantage of this technique is that exposure to water has been known to cause damage to the fiber over time. Specifically, such exposure to water can degrade the attenuation properties of the fiber to light passing through it. This effect is typically non-linear, which generates inaccuracies in some measured parameters. Increased attenuation, if severe enough, may cause loss of the signal to be measured.